A. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device in which method an electrode is formed by an electroless plating method on a semiconductor device as a rectifying element such as a diode.
B. Description of the Related Art
In an electric power converter device for converting direct current to alternating current or vice versa such as an inverter or a converter, a rectifying element formed of a single element semiconductor chip is used as a free wheeling diode in being connected to a power semiconductor element in inverse parallel for current free wheeling.
A cross section of a unit cell structure of the diode is shown in FIG. 8. In FIG. 8, on one of surfaces of N-type silicon substrate 1, P-type diffused layer 2 is selectively formed. Around a perimeter of the surface, insulator film 3 such as an oxide film is formed so as to surround P-type diffused layer 2. On the surface of P-type diffused layer 2, aluminum electrode 4 is formed as an anode electrode so that an aluminum wire is easily bonded thereto. Furthermore, in a planar structure easily complying with a wafer on a trend toward an enlarged diameter, a plurality of ring-like P-type guard ring regions are formed on a wafer so as to surround respective P-type diffused layer 2 in active regions. In contact with each of the guard ring regions, a metal layer is formed as a voltage withstanding structure which layer is a field plate spreading toward the perimeter on the insulator film. The metal layer in the voltage withstanding structure section becomes open electrode 5 that is connected to no external circuit. Moreover, on the other surface, a cathode electrode is formed. The electrode is generally formed of a plurality of films made up of Ti layer 6, Ni layer 7 and Au layer 8 in the order from the silicon substrate side so that soldering is easily carried out on the surface. In addition, on the anode side surface of the silicon substrate, scribe lines are scored in a lattice for separating the silicon substrate into chips of rectifying elements after the rectifying elements are formed. Each of the scribe lines is provided on P-type diffused layer 2 and further reaches P-type diffused layer 2 through open electrode 5 to be at a potential of the anode.
The diode is operated as follows. An applied potential to the anode side as being higher than that to the cathode side brings the diode into conduction, and in reverse, the applied potential to the anode side as being lower than the cathode side brings the diode out of conduction, which makes the diode operated as being a rectifying element.
For streamlining a process when manufacturing the rectifying element, there is a requirement of forming the metal film on the electrode by soldering that can treat large numbers of electrodes at one time. Furthermore, for streamlining a process when assembling the rectifying element into a module or a molded package, there is also a requirement of carrying out connection of a lead terminal to the electrode on the anode side by soldering instead of wire bonding. When soldering the lead terminal to the electrode on the anode side, it is necessary to apply Ni also to the aluminum face on the anode side, since solder will not adhere to the aluminum face. One such Ni application method entails selectively forming an electrode by electroless plating (see JP-A-2000-216410, JP-A-11-17197, JP-A-5-291186, and JP-A-5-335600).
A cross sectional view of a unit cell structure of a diode with electrodes formed by such a method is presented in FIG. 9. In FIG. 9, on a surface of aluminum electrode 4 as an anode electrode, Ni metallic deposit 9 is formed. In general, however, in order to prevent Ni metallic deposit 9 from being oxidized, Au metallic deposit 10 is further formed thereon.
Since the metallic deposits are formed on metal electrodes, the metallic deposits also may form on a surface of open electrode 5 and the bottom surface of the cathode besides the surface of aluminum electrode 4 to be the anode electrode. The metallic deposits formed also on the surface of open electrode 5 and the bottom surface of the cathode cause the metal electrode films to be thickened. This increases the effect of the difference in coefficient of thermal expansion between the metal electrode film and N-type silicon substrate 1, causing an increased amount of warping of the wafer.
Moreover, as shown in a cross sectional view in FIG. 10, there is further requirement of stably forming Ni metallic deposit 9 and Au metallic deposit 10 on not only the anode electrode side but also the cathode electrode side. In this case, between Ni metallic deposit 9 and N-type silicon substrate 1, a metal electrode such as aluminum electrode 4 intervenes.
Furthermore, a plurality of diode regions are formed on a silicon substrate and connected to external circuits except diode regions with faulty characteristics (see JP-UM-B-5-47479 and JP-B-5-57744).
Vapor deposition can be also be used to form the metal electrode. In vapor deposition, a barrier can be provided on Ni as a substrate to make it possible to stop growth of an alloy of Ni and applied solder in Ni metallic deposit 9. Therefore, Ni metallic deposit 9 can be as thin as about 0.7 μm. However, from the standpoint of ease of formation of a metal electrode film onto a face with a complicated form having steps or from the standpoint of mass productivity, vapor deposition is incomparably inferior to electroless plating. Thus, electroless plating is desirable for formation of electrodes from the view point of cost reduction by mass production. In spite of this, Ni metallic deposit 9 formed by electroless plating is inferior in its ability to prevent growth of an alloy of Ni and applied solder as compared with vapor deposition, so that Ni metallic deposit 9 must have a thickness of as much as about 3 μm. Therefore, when electroless plating is used, plating on the bottom surface of the silicon substrate, or other areas where plating is not desired, produces a thick metallic deposit that increases the effect of a difference in coefficient of thermal expansion between the thick metallic deposit and the silicon substrate. This results in an increase in an amount of warping of the wafer to produce a problem of breaking the silicon substrate due to resulting stress in the worst case.
In order to prevent the increase in stress, an oxide film is left on the bottom face of the silicon substrate or the bottom face is covered with a tape to keep the bottom face free from adhesion of Ni. This, however, results in extra process steps by the additional step of removing the oxide film or stripping the tape, as well as possible lack of strength of the silicon substrate at tape stripping, and furthermore, degradation in film quality due to film contamination by an adhesive of the tape dissolved into plating solution.
In electroless plating of Ni, metallic ions Ni+ positively charged in plating solution adhere to material to be plated due to slight electric potential difference between the metallic ions and the material to be plated, and grow as metallic Ni. In a rectifying element in which a P—N junction is formed, a P-type section is negatively charged and an N-type section is positively charged by a diffusion potential at the P—N junction. Therefore, the metallic ions tend to adhere easily onto an anode electrode of P-type to form a metallic deposit. Observation of situation of adhesion of a metallic deposit reveals that the metallic deposit grows only on the surface of the anode side in an early stage in which no short circuit is formed between the anode side and the cathode side. At this time, the metallic deposit also grows on a scribe line at the same potential as that of the anode. After a while, as the metallic deposit grows on the scribe line and then also on the perimeter of the wafer, the anode and the cathode are short-circuited by the metallic deposits grown via the perimeter of the wafer, and the metallic deposit grows immediately also on the cathode side. This immediately grown metallic deposit results in a very unstable film of metallic deposit with a variation in film thickness.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.